Micromachined horn

ABSTRACT

An acoustic device includes a transducer formed on a first surface of a substrate and an acoustic horn formed in the substrate by a dry-etching process through an opposing second surface of the substrate. The acoustic horn is positioned to amplify sound waves from the transducer and defines a non-linear cross-sectional profile.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of application Ser. No.12/434,092 filed on May 1, 2009, which is hereby incorporated byreference for all purposes.

BACKGROUND

Generally, acoustic transducers convert received electrical signals toacoustic signals when operating in a transmit mode, and/or convertreceived acoustic signals to electrical signals when operating in areceive mode. The functional relationship between the electrical andacoustic signals of an acoustic transducer depends, in part, on theacoustic transducer's operating parameters, such as natural or resonantfrequency, acoustic receive sensitivity, acoustic transmit output powerand the like.

A horn is an acoustic waveguide which provides an efficient means ofcoupling a sound source to the environment. Generally, horns may be usedto amplify acoustic waves, as indicated by incorporation of horns invarious acoustic devices such as loudspeakers and musical instrument,for example, to increase their sound output. Sound (acoustic waves)enters the horn at the throat and exits the horn at the mouth. Inaddition, a horn may be used to modify directionality characteristics orradiation pattern of an acoustic emitter, e.g., by the location, sizeand shape of the horn.

By extension, acoustic horns may be used with micromachined acoustictransducers, such as such as piezoelectric ultrasonic transducers andmicro micro-electro-mechanical system (MEMS) transducers. Whenimplemented on a small scale, an acoustic horn may be etched into asilicon substrate, for example, using a wet etchant, such as potassiumhydroxide (KOH), which etches silicon preferentially along variouscrystal planes. However, KOH is an anisotropic etching process whichproduces limited results with respect to horn characteristics, as shownin the schematic diagram of FIG. 1. In particular, KOH etching producesa pyramidic shaped horn 110 in a substrate 105. The horn 110 has linearcross-sections and a square mouth, opening on an outside (e.g., top)surface of the substrate 105. Notably, an angle 112 defined by the mouthof the horn 110 and the outer surface of the substrate 105 isnecessarily fixed at 54.7 degrees. These limitations on size and shapeof micro machined acoustic horns constrain design flexibility.

SUMMARY

In a representative embodiment, an acoustic device includes a transducerformed on a first surface of a substrate and an acoustic horn formed inthe substrate by a dry-etching process through an opposing secondsurface of the substrate. The acoustic horn is positioned to amplifysound waves from the transducer and defines a non-linear cross-sectionalprofile.

In a representative embodiment, a method of fabricating an integratedacoustic device includes forming a transducer on a front side of asubstrate, and dry-etching an acoustic horn through a back side of thesubstrate. A throat of the acoustic horn is positioned adjacent to theacoustic transducer and a cross-section of the acoustic horn hasnon-linear sidewalls.

In a representative embodiment, an acoustic device includes asemiconductor substrate, a piezoelectric ultrasonic transducer and anacoustic horn. The piezoelectric ultrasonic transducer is formed on afront surface of the substrate. The acoustic horn is formed through aback surface of the substrate by a deep reactive ion etching (DRIE)dry-etching process, a throat of the acoustic horn being positionedadjacent to the transducer. A cross-sectional profile of the acoustichorn includes substantially exponential sidewalls.

BRIEF DESCRIPTION OF THE DRAWINGS

The example embodiments are best understood from the following detaileddescription when read with the accompanying drawing figures. It isemphasized that the various features are not necessarily drawn to scale.In fact, the dimensions may be arbitrarily increased or decreased forclarity of discussion. Wherever applicable and practical, like referencenumerals refer to like elements.

FIG. 1 is a cross-sectional diagram of a conventional horn of anacoustic device.

FIG. 2 is a cross-sectional diagram of an acoustic horn of an acousticdevice, according to a representative embodiment.

FIG. 3 is a cross-sectional diagram of an acoustic horn of an acousticdevice, according to a representative embodiment.

FIG. 4 is a cross-sectional diagram of an acoustic horn of an acousticdevice, according to a representative embodiment.

FIG. 5 is a cross-sectional diagram of an acoustic horn of an acousticdevice, according to a representative embodiment.

FIG. 6 is a cross-sectional diagram of an acoustic horn of an acousticdevice, according to a representative embodiment.

FIG. 7 is a cross-sectional diagram of an acoustic horn of an acousticdevice having a thin film coating, according to a representativeembodiment.

FIGS. 8A, 8B and 8C are cross-sectional diagrams of mouths of acoustichorns, according to representative embodiments.

FIG. 9 is a cross-sectional diagram of an acoustic horn of an acousticdevice, according to a representative embodiment.

FIG. 10 is a cross-sectional diagram of an acoustic horn of an acousticdevice, according to a representative embodiment.

DETAILED DESCRIPTION

In the following detailed description, for purposes of explanation andnot limitation, representative embodiments disclosing specific detailsare set forth in order to provide a thorough understanding of thepresent teachings. However, it will be apparent to one having ordinaryskill in the art having had the benefit of the present disclosure thatother embodiments according to the present teachings that depart fromthe specific details disclosed herein remain within the scope of theappended claims. Moreover, descriptions of well-known apparatuses andmethods may be omitted so as to not obscure the description of therepresentative embodiments. Such methods and apparatuses are clearlywithin the scope of the present teachings.

Furthermore, as used herein, the term “acoustic” encompasses sonic,ultrasonic, and infrasonic. For example, a transmitting acoustictransducer may transmit sonic, and/or ultrasonic, and/or infrasonicwaves. Also, unless otherwise noted, when a first device is said to beconnected to, or coupled to, a node, signal, or second device, thisencompasses cases where one or more intervening or intermediate devicesmay be employed to connect or couple the first device to the node,signal, or second device. However, when a first device is said to be“directly connected” or “directly coupled” to a node, signal, or seconddevice, then it is understood that the first device is connected orcoupled to the node, signal, or second device without any intervening orintermediate devices interposed therebetween.

FIG. 2 is a cross-sectional diagram of an acoustic horn for an acousticdevice, according to a representative embodiment. As shown in FIG. 2,acoustic device 200 includes a micromachined acoustic horn 210 formed bya back-side dry-etching process in a substrate 215, on which an acoustictransducer 220 has been formed.

In an embodiment, the acoustic transducer 220 includes a piezoelectricthin film resonator, such as a piezoelectric MEMS ultrasonic transducer(PMUT) or a film bulk acoustic resonator (FBAR) device, for example.However, the acoustic transducer 220 in FIG. 2 (as well as the acoustictransducers depicted in FIGS. 3-7 and 9-10, below) is intended to berepresentative of any type of acoustic component or MEMS device, such asa capacitive or piezoresistive device, without departing from the scopeof the present teachings. The acoustic transducer 220 may also representmultiple transducers, indicating that the acoustic horn 210 may formedand used in conjunction with a transducer array, in an embodiment.

In various embodiments, the acoustic transducer 220 is capable ofoperating in transmit and/or receive modes. When operating in thetransmit mode, an excitation signal is received by the acoustictransducer 220, which outputs a corresponding acoustic signal accordingto a predetermined function as the transducer response, generated bymechanical vibrations induced by the received electrical excitationsignal. When operating in the receive mode, an excitation signal is anacoustic signal received by the acoustic transducer 220, which outputs acorresponding electronic signal as the transducer response.

According to an embodiment, the fabrication process begins with a waferor substrate 215, which is a semiconductor substrate, such as silicon,gallium arsenide (GaAs), a transparent substrate, such as glass, orother suitable substrate material. The acoustic transducer 220 is formedon a front or top surface of the substrate 215 using a “swimming pool”method, for example, as described in U.S. Pat. No. 7,275,292, to Ruby etal., issued Oct. 2, 2007, the contents of which are hereby incorporatedby reference. That is, the front surface of the substrate 215 isinitially etched to form a “swimming pool” having a size and shape ofthe desired transducer 220, using any wet and/or dry-etching technique.A sacrificial etch stop material, such as phosphosilicate-glass (PSG),is deposited in the etched portion to form etch stop 221. The frontsurface of the substrate 215 and the exposed portion of the etch stop221 may then be polished, for example, using a chemical mechanicalpolish (CMP). In an alternative embodiment, the acoustic transducer 220itself may serve as the etch stop.

The acoustic transducer 220 is then formed on the etch stop 221 usingany of a variety of layering techniques. For example, a first metallayer 223 (first electrode) is formed on the etch stop 221, apiezoelectric material layer 224 is formed on the metal layer 223, and asecond metal layer 225 (second electrode) is formed on the piezoelectricmaterial layer 224. The first and second metal layers 223 and 225 may beformed of any metal compatible with semiconductor processes, such asmolybdenum, tungsten or aluminum, for example. The piezoelectricmaterial layer 224 may be formed of a material such as aluminum nitride,lead zirconate titanate (PZT), for example, or other film compatiblewith semiconductor processes. In various embodiments, the first metallayer 223, the piezoelectric material layer 224 and the second metallayer 225 are sequentially formed on the front surface of the substrate210, and then etched using an etch mask to provide the acoustictransducer 220 of a desired size and shape, positioned over the etchstop 221, as shown in FIG. 2. However, other layering and shapingtechniques may be incorporated without departing from the scope of thepresent teachings.

The acoustic horn 210 is formed by a horn etching process, performed ona back or bottom surface of the substrate 215, according to arepresentative embodiment, using the previously formed etch stop 221 asan etch stop barrier. As stated above, the horn etching process isperformed by dry-etching the back surface of the substrate 215 in acontrolled manner to generate a unique shape of the acoustic horn 210,e.g., other than the conventional square pyramid shape resulting fromwet etching, as shown in FIG. 1, for example. Dry-etching processesinclude non-plasma and plasma based dry-etching. In plasma baseddry-etching, radio frequency (RF) signals drive chemical reactions ofgases to create plasma, e.g., at high temperatures. Types of plasmabased dry-etching include physical and chemical etching, reactive ionetching (RIE) and deep reactive ion etching (DRIE).

One type of DRIE is referred to the Bosch® etching process, an exampleof which is described in U.S. Pat. No. 5,501,893, issued Mar. 26, 1996,the contents of which are hereby incorporated by reference. Generally,the Bosch® etching process uses high density plasma that cycles betweena plasma-etching process and a teflon-coating process, which deposits anetch-resistant polymer (e.g., carbon) on side walls to prevent lateraletching. That is, the teflon-coating process periodically coats etchwalls, exposed by the plasma-etching process, so that subsequentplasma-etching bores deeper without increasing the width of the etch.

The Bosch® etching process uses sulfur hexafluoride SF₆ for theplasma-etching process. However, according to various embodiments, otherplasmas may be used to etch the backside of the substrate 215, such asfluorine plasma (e.g., CF₄ or SF₆), chlorine plasma (e.g., Cl₂), ormixed chlorine and fluorine plasma (e.g., Cl₂+SF₆). Also, a fluorine andcarbon compound may be used, where the fluorine content is greater thancarbon (e.g., C₄F₈). The shape of the acoustic horn 210 may then bycustomized by dynamically adjusting the ratio of elements in thecompound, the time of exposure, pressure and/or the concentration of thecompound. For example, as the portion of carbon is increased in afluorine and carbon compound, the etch becomes less lateral. An exampleof the process for etching various horn shapes is discussed in detailwith respect to FIG. 4, below.

As stated above, the Bosch® etching process alternates between etch anddeposition cycles. For instance, the etching process may use a sixsecond etch cycle with SF₆ gas, followed by a two second depositioncycle using C₄F₈ gas. These cycles are repeated to create a verticaletch with straight sidewalls, for example. By adjusting the relativepulse times of the etch and deposition cycles, the sidewall angle can bealtered. For example, increasing the deposition cycle to four secondswhile keeping the etch cycle at six seconds creates a closing profile(i.e., the diameter of the via narrows as the etch progresses).

FIG. 3 is a cross-sectional diagram of an acoustic horn for an acousticdevice, according to another representative embodiment. As shown in FIG.3, acoustic device 300 includes a micromachined acoustic horn 310 formedby a back side dry-etching process in a substrate 315, on which anacoustic transducer 320 has been formed. As discussed above, theacoustic transducer 320 is depicted as a piezoelectric thin filmresonator, such as a PMUT device, although it may be any type of MEMSdevice.

According to an embodiment, the fabrication process begins with asilicon-on-insulator (SOI) wafer, for example, including a bulksubstrate 315, a buried oxide layer 316 and a device layer 317. The bulksubstrate 315 and the device layer 317 may be formed of silicon or GaAs,and the buried oxide layer 316 may be formed of silicon dioxide (SiO₂),for example. In the depicted embodiment, the buried oxide layer 316serves as an etch stop with respect to the subsequent back side etchingprocess to form the acoustic horn 310. In various embodiments, thedevice layer 317 may be part of the device 300, as shown, or it may beetched away.

The acoustic transducer 320 is then formed on the device layer 317 usingany of a variety of layering techniques. For example, a first metallayer 323 (first electrode) is formed on the device layer 317, apiezoelectric material layer 324 is formed on the metal layer 323, and asecond metal layer 325 (second electrode) is formed on the piezoelectricmaterial layer 324. The first and second metal layers 323 and 325 may beformed of any metal compatible with semiconductor processes, such asmolybdenum, tungsten or aluminum, for example. The piezoelectricmaterial layer 324 may be formed of a material such as aluminum nitride,lead zirconate titanate (PZT), for example, or other film compatiblewith semiconductor processes. In various embodiments, the first metallayer 323, the piezoelectric material layer 324 and the second metallayer 325 are sequentially formed on the front surface of the devicelayer 317, and then etched using an etch mask to provide the transducer320 of a desired size and shape, as shown in FIG. 3. However, otherlayering and shaping techniques may be incorporated without departingfrom the scope of the present teachings.

The acoustic horn 310 is formed by an etching process performed on aback or bottom surface of the bulk substrate 315, according to arepresentative embodiment, using the previously formed buried oxidelayer 316 as an etch stop barrier. As stated above, the horn etchingprocess is performed by dry-etching the back surface of the bulksubstrate 315 in a controlled manner to generate a unique shape of theacoustic horn 310, e.g., other than the conventional square pyramidshape resulting from wet etching, as shown in FIG. 1, for example. Theprocess of etching various horn shapes is discussed in detail withrespect to FIG. 4, below.

According to various embodiments, the sizes and shapes of acoustic horns210 and 310 may vary to provide unique benefits for any particularsituation or to meet application specific design requirements of variousimplementations, as would be apparent to one skilled in the art. Forexample, FIG. 4 is a cross-sectional diagram of an acoustic horn for anacoustic device, according to a representative embodiment, in which theacoustic horn has a generally hyperbolic or exponential cross-sectionalshape.

More particularly, acoustic horn 410 of acoustic device 400 includes anarrow throat 411 that is adjacent to a transducer device 420, such as apiezoelectric thin film resonator, and a wide or flared mouth 413. Whenthe acoustic device 420 is a sound emitting acoustic transducer, forexample, sound waves generated by the acoustic device 420 in response toan electric excitation signal enter the acoustic horn 410 at the throat411 and exit as amplified sound waves from the mouth 413. Notably, theacoustic horn 410 is depicted facing upward from the acoustic device 420for convenience of explanation. However, it is understood that theacoustic horn 410 may be back etched in the substrate 415 afterformation of the transducer device 420 on a front surface of thesubstrate 415, as described above with respect to FIGS. 2 and 3.

As shown in FIG. 4, an exponential shape of the acoustic horn 410 can beapproximated by a series of dry-etches, each with different conditions.An exponential horn has a varying cross-sectional area, A(x), given byEquation (1):A(x)=A₀e^(mx)   Equation (1)

In Equation (1), A₀ is the area of the throat 411, m is a flareconstant, and x is the length of the horn (as measured perpendicularlyfrom a plane containing the throat 411 to a plane containing the mouth413). Compared to a conical horn (e.g., an example of which is shown inFIG. 6), an exponential horn has a flatter frequency response, above thecutoff frequency f_(c). Also, the exponential acoustic horn 410 enablestransmission of more sound, while reducing thickness requirements of thesubstrate 415. However, a conical horn may be constructed using a singledry-etch with a constant angle, where an exponential horn requires amulti-step dry-etch, as discussed below.

In order to determine the etching process, the parameters of theacoustic horn 410 are first determined. For example, the acoustic horn410 may be designed for frequencies above 50 kHz. To provide suitabletransmission, the flare constant m may be set to 1500, for example. Thediameter of the throat 411 is determined by the diameter of the acousticdevice 420, which is about 1.0 mm for a typical MEMS transducer. Thesubstrate 415 may have a thickness of about 0.7 mm, for example.Therefore, applying Equation (1) to the illustrative parameters, thediameter of the throat 411 is 1.0 mm and the diameter of the mouth 413is calculated to be 1.7 mm.

In the depicted embodiment, the acoustic horn 410 is etched in threeconsecutive stages to approximate an exponential shape. It is understoodthat additional etching stages may be performed, resulting in a hornshape that more nearly approximates a true exponential shape. In anembodiment, the etching stages may incorporate the Bosch® etchingprocess, for example. The Bosch® etching process using alternating gasflows of a fluorine containing gas and a fluoro-carbon, as discussedabove. For example, the etching process may alternate between an SF₆cycle for etching a silicon trench and a C₄F₈ cycle for periodicallydepositing polymer on the exposed sidewalls to prevent subsequentlateral etching. The parameters of the Bosch® etching process may bemodified to create a variety of trench profiles, such as a closingprofile or an opening profile (i.e., the diameter of the via widens asthe etch progresses). A customized closing profile is created bymodifying different etch parameters, such as increasing pressure,lowering bias power, lowering source power and/or lowering the SF₆ toC₄F₈ flow ratio. Similarly, a customized opening profile may be createdby modifying these parameters in an opposite manner, such as decreasingpressure, increasing bias power, increasing source power and/orincreasing the SF₆ to C₄F₈ flow ratio.

Referring again to FIG. 4, to form the acoustic horn 410 having anapproximate exponential shape, the dry-etching process starts at themouth 413 with a set of initial closing profile conditions. The closingprofile conditions are dynamically adjusted as the dry-etching proceedsthrough first, second and third sections 430, 440 and 450 of thesubstrate 415 toward the throat 411. For example, in the first substratesection 430, the ratio of SF₆ to C₄F₈ pulse times is relatively low,e.g., values between about 1.0 to about 2.0, and the bias power is setrelatively low, e.g., at about 50 W. Accordingly, the dry-etchingcreates a first sidewall of the acoustic horn 410 (having a circularcross-section, for example, as shown in FIG. 8A) having an angle ofabout 48 degrees to a depth of approximately 0.1 mm. As the dry-etchingprogresses to the second section 440, the SF₆/C₄F₈ pulse ratio isincreased, e.g., to values between about 2.0 to about 5.0, and the biaspower is increased, e.g., to about 100 W. Accordingly, the dry-etchingcreates a second sidewall of the acoustic horn 410 (having the samerelative cross-section) having an angle of about 57 degrees to anadditional depth of approximately 0.3 mm. In the third section 450, theSF₆/C₄F₈ pulse ratio is again increased to exceed about 5.0, and thebias power is increased, e.g., to about 150 W, to create a near verticalprofile. Accordingly, the dry-etching creates a third sidewall of theacoustic horn 410 (having the same relative cross-section) having anangle of about 64 degrees to an additional depth of approximately 0.3mm.

It is understood that the ratios discussed above are only forillustrative purposes, and that different etch tools require differentparameters. It is further understood that the number of substratesections (e.g., first through third substrate sections 430, 440 and450), as well as the thickness and angle of each substrate section, mayvary to provide unique benefits for any particular situation or to meetapplication specific design requirements of various implementations, aswould be apparent to one skilled in the art. For example, as discussedabove, as the number of substrate sections increases, the overall shapeof the acoustic horn 410 more nearly approximates a true exponentialcurve.

FIGS. 5 and 6 are cross-sectional diagrams of acoustic horns havingvarious non-limiting shapes, according to representative embodiments.

FIG. 5 shows acoustic device 500, which includes an acoustic horn 510formed in substrate 515 for amplifying sound waves of acoustictransducer 510. The acoustic horn 510 is dry-etched to have a steppedprofile with two distinct angles. A stepped profile provides advantagesof a horn shape having small and large angles, without the addedcomplexity and etching steps required to produce an exponential hornshape, as discussed above with respect to FIG. 4. For example, theacoustic horn 510 may be dry-etched using a closing profile with onlytwo sets of sequentially applied closing profile parameters. Inaddition, the representative embodiment of FIG. 6 may be used to realizea “constant directivity” horn in a micromachined substrate. This classof acoustic horns maintains a directivity pattern over a wide range offrequencies.

FIG. 6 shows acoustic device 600, which includes an acoustic horn 610 or620 formed in a substrate 615 for amplifying sound waves of the acoustictransducer 610. The acoustic horn 610 or 620 is dry-etched to have aconical profile at any single angle. When the cross-section of the mouthon the outside surface of the substrate 615 is circular (e.g., as shownin FIG. 8A), the shape of the acoustic horns 610 and 611 is conical.

The representative acoustic horn 610 has sidewalls formed at a firstangle θ₁ with a surface of the substrate 615, while the representativeacoustic horn 611 has sidewalls formed at a second angle θ₂ with thesurface of the substrate 615. The first angle θ₁ is smaller than thesecond angle θ₂, indicating that the sidewalls of the acoustic horn 610are more vertical than the sidewalls of the acoustic horn 611. The smallfirst angle θ₁ improves sound transmission through the acoustic horn 610in a relatively narrow direction, thus increasing the magnitude of theradiated sound, which is especially helpful for low frequencies. Incomparison, the larger second angle θ₂ has a lower magnitude of radiatedsound, and thus is used to achieve a desired mouth diameter of theacoustic horn 611 in a thinner substrate 615. As described above, Inorder to produce the larger second angle θ₂, the dry-etching processrequires a lower SF₆/C₄F₈ pulse ratio, a lower bias power, a lowersource power and/or a lower application pressure than the dry-etchingprocess used to produce the first angle θ₁.

Notably, the acoustic horns 510 and 610/611 are depicted facing upwardfrom the acoustic device 520 and 620, respectively, for convenience ofexplanation. However, it is understood that the acoustic horns 510 and610/611 may be back etched in the substrates 515 and 615, respectively,as described above with respected to FIGS. 2 and 3.

FIG. 7 is a cross-sectional diagram of an acoustic horn of an acousticdevice having a thin film coating, according to a representativeembodiment.

FIG. 7 shows acoustic device 700, which includes a micromachinedacoustic horn 710 formed in a substrate 715 for amplifying sound wavesof the acoustic transducer 710. For any horn geometry discussed herein,including the acoustic horn 720, a thin film coating 718 may be appliedto improve sound transmission and/or environmental robustness. Forexample, following completion of the dry-etching process, the substrate715 within the acoustic horn 720 may be coated with a thin film coating718 formed of very hard material, such as diamond, to reduce soundabsorption by the substrate 715. In another representative embodiment,the exposed substrate 715 within the acoustic horn 720 may be coatedwith a thin film coating 718 formed of hydrophobic material, such assilicon nitride, to reduce the collection of water droplets on thesubstrate sidewalls of the acoustic horn 720. This is particularlyuseful in a high-humidity environment, for example. In addition, inanother representative embodiment, the thin film coating 718 may beformed of an anti-corrosive material, such as silicon carbide, toprovide resistance to corrosive environments.

FIGS. 8A, 8B and 8C are cross-sectional diagrams of mouths of acoustichorns, according to representative embodiments, indicating examples ofthe various shaped horn mouths and/or corresponding horn throats (notshown).

FIG. 8A depicts a circular cross-section of a micromachined horn mouth813, exposed at an outer surface of a substrate 823, which may begenerated pursuant to the dry-etching process depicted in FIGS. 4-7. Forexample, an acoustic horn 410 having generally exponential sidewalls mayhave a circular mouth 413 and an acoustic horn 510 having steppedsidewalls, may have circular mouths, as shown by micromachined hornmouth 813. Likewise, a generally conical acoustic horn, such as acoustichorns 610 and 611 of FIG. 6, may have circular mouths. FIG. 8B depictsan elliptical cross-section of a micromachined horn mouth 814, exposedat an outer surface of the substrate 824, which may be generatedpursuant to the dry-etching process depicted in FIGS. 4-5.

FIG. 8C depicts a representative irregularly shaped cross-section of amicromachined horn mouth 815, exposed at an outer surface of a substrate825. The micromachined horn mouth 815 depicts one example of innumerablepossible irregularly shaped cross-sections, indicating the versatilityof dry-etching process for acoustic horns. The irregular shaped hornmouth 815 may be included in an acoustic horn have irregular sidewalls.

For example, referring to FIG. 9, representative acoustic horn 910 ofacoustic device 900, formed in a substrate 915 for amplifying soundwaves of the acoustic transducer 910, has skewed sidewalls. Accordingly,the acoustic horn 910 may have an irregularly shaped mouth 913 and/orthroat 911, similar to the micromachined horn mouth 830 of FIG. 8C, forexample.

In order to achieve the irregular cross-section of the acoustic horn910, for example, a modified dry etch and mask process is used. Anotherillustrative method of creating sloped sidewall etches (other thanmodified Bosch® etching processes, as discussed above, for example) isto make use of resist erosion during the etch. By introducing O₂ intothe etch plasma, the sidewall of the resist is consumed during the etchand erodes to create a larger opening. As the etch progresses and theresist erosion becomes more pronounced, the top of the via (nearest tothe resist) becomes progressively larger to create a closing profile. Inorder to skew the sidewalls, such that the angle of the two oppositesides of the via are different (e.g., as shown in FIG. 8C and/or FIG.9), a hard mask may be placed on one side of the via opening. Forinstance, a material that is non-etchable in the plasma chemistry couldbe placed on one side of the via opening instead of resist. In the caseof SF₆ chemistry, the material may include Au or AlN, for example, whichare referred to as a hard mask. The hard mask would not be attacked bythe etch plasma and thus would not erode. The net effect would be a nearvertical sidewall under the hard mask and a sloped sidewall on the sidewith the resist. The sidewall angle under the resist may be controlledby varying the etch parameters. For example, more O₂ and higher sourcepowers cause more erosion and more sloped sidewalls.

The dry-etching process may be incorporated into formation of varioustypes of MEMS acoustic devices, including for example a microcapacoustic transducer device, described in U.S. patent application Ser.No. 12/430,966 to Philliber et al., filed Apr. 28, 2009, the contents ofwhich are hereby incorporated by reference.

For example, FIG. 10 shows a representative microcap acoustic device1000, which includes acoustic transducer 1020 formed on a front surfaceof lower wafer or substrate 1015, such that the acoustic transducer 1020is contained within cavity 1055 formed by lower substrate 1015 (e.g.,“base wafer”), upper wafer or substrate 1016 (e.g., “cap wafer”) andgasket 1050. An acoustic horn 1010 is dry-etched in the back or bottomsurface of the lower substrate 1015, according to the variousrepresentative embodiments described herein (before or after formationof the cavity 1055). In the depicted embodiment, the acoustic horn 1010has a conical profile, although it is understood that the acoustic horn1010 may have any profile according to the flexible dry-etching processdescribed herein.

Generally, in various embodiments, the lower substrate 1015 and/or uppersubstrate 1016 are semiconductor wafers, such as silicon or GaAs, ortransparent substrates, such as glass. Beneficially, however, the lowerand upper substrates 1015 and 1016 are made of the same material toavoid thermal expansion mismatch problems. The gasket 1050 bonds thelower and upper substrates 1015 and 1016 to define the cavity 1055. Thegasket 1050 may be fabricated directly onto one of bonded lower andupper substrates 1015 and 1016, or can be applied during the bondingprocess. The gasket 1050 may be made of silicon applied to one of thelower or and upper substrates 1015 and 1016, although other material maybe used, including polymers (BCB, Polyimide, etc.) or various metals ormetallic alloys (Au, Cu, Au—Hg alloy, etc.), for example.

In an embodiment, the gasket 1050 hermetically seals the cavity 1055between the lower and upper substrates 1015 and 1016. In anotherembodiment, the gasket 1050 may have a structure which permits air flowto pass between the exterior of the acoustic device 1000 and the cavity1055, which at the same time inhibits or prevents contaminates fromentering the cavity 1055 and coming in contact with the acoustictransducer 1020.

Accordingly, by adjusting parameters during a dry-etching process of asemiconductor substrate (e.g., silicon wafer), the shapes and angles ofacoustic horn sidewalls are altered to obtain any variety of hornshapes. For example, an acoustic horn shape may be created by startingwith a wide feature opening in the mask and using a closing profile togenerate the outermost portion of the horn. As the etch progressesthrough the semiconductor substrate, the parameters may be adjusted tocreate a more vertical profile, such that the etch is vertical by thetime it reaches the position of the acoustic device (e.g., transducer)on the opposite side of the semiconductor substrate. The dry-etchingprocess may be refined by any number of steps (2-∞) to create a smoothsidewall.

The various components, materials, structures and parameters areincluded by way of illustration and example only and not in any limitingsense. In view of this disclosure, those skilled in the art canimplement the present teachings in determining their own applicationsand needed components, materials, structures and equipment to implementthese applications, while remaining within the scope of the appendedclaims.

The invention claimed is:
 1. An acoustic device, comprising: atransducer formed on a first surface of a substrate; and an acoustichorn formed in the substrate by a dry-etching process through anopposing second surface of the substrate, and positioned to amplifysound waves from the transducer, the acoustic horn defining anon-linear, asymmetrical cross-sectional profile.
 2. The acoustic deviceof claim 1, wherein the cross-sectional profile comprises substantiallyexponential sidewalls.
 3. The acoustic device of claim 1, wherein thecross-sectional profile comprises stepped sidewalls.
 4. The acousticdevice of claim 1, wherein the acoustic horn defines a mouth having ashape other than a square shape.
 5. The acoustic device of claim 1,wherein the shape of the mouth comprises one of a circle or an oval. 6.The acoustic device of claim 1, wherein the transducer comprises apiezoelectric MEMS ultrasonic transducer (PMUT).
 7. The acoustic deviceof claim 1, wherein the transducer comprises a piezoelectric thin filmbetween two metal layers.
 8. The acoustic device of claim 1, furthercomprising: a thin film coated on interior walls of the acoustic horn.9. The acoustic device of claim 8, wherein the thin film comprisesdiamond for reducing sound absorption of the substrate within theacoustic horn.
 10. The acoustic device of claim 8, wherein the thin filmcomprises one of silicon nitride and silicon carbide.
 11. An acousticdevice, comprising: a semiconductor substrate; a piezoelectricultrasonic transducer formed on a front surface of the substrate; and anacoustic horn formed through a back surface of the substrate by amulti-step dryetching process, a throat of the acoustic horn beingpositioned adjacent to the transducer, wherein a cross-sectional profileof the acoustic horn comprises substantially exponential sidewalls. 12.The acoustic device of claim 11, wherein the ultrasonic transducercomprises a piezoelectric MEMS ultrasonic transducer (PMUT).
 13. Theacoustic device of claim 11, wherein the ultrasonic transducer comprisesa piezoelectric thin film between two metal layers.
 14. The acousticdevice of claim 11, further comprising: a thin film coated on interiorwalls of the acoustic horn.
 15. The acoustic device of claim 14, whereinthe thin film comprises diamond for reducing sound absorption of thesubstrate within the acoustic horn.
 16. The acoustic device of claim 14,wherein the thin film comprises one of silicon nitride and siliconcarbide.
 17. An acoustic device comprising: a semiconductor substrate; apiezoelectric ultrasonic transducer formed on a front surface of thesubstrate; and an acoustic horn formed through a back surface of thesubstrate by a dry-etching process, a throat of the acoustic horn beingpositioned adjacent to the transducer, wherein a cross-sectional profileof the acoustic horn comprises stepped sidewalls with at least twodistinct angles, wherein the stepped sidewalls of the acoustic horn areskewed with respect to one another, such that corresponding angles ofopposite sides of the stepped sidewalls are different.
 18. The acousticdevice of claim 17, further comprising: a thin film coated on thestepped sidewalls of the acoustic horn, the thin film comprising one ofdiamond, silicon nitride and silicon carbide.